1. Field of the Invention
The present invention relates to bismuth vanadate pigments comprising at least one coating containing calcium fluoride, bismuth oxyfluoride or a lanthanide fluoride or oxyfluoride or a mixture thereof.
This invention further relates to the use of these bismuth vanadate pigments for coloring paints, printing inks and plastics.
2. Description of the Background
Bismuth vanadate pigments are well known. As well as the pure BiVO4 pigment, there are a number of BiVO4 pigments in which some of the metal and/or oxygen atoms are replaced by other metals and/or nonmetals. These pigments are useful nontoxic yellow pigments and are particularly suitable for coloring paints and plastics. To improve their application properties, especially their thermal stability, their weatherfastness and their resistance to chemicals, bismuth vanadate pigments are frequently provided with protective coats of metal oxides (including silicates) and/or protective coats of phosphates with or without fluoride.
For instance, U.S. Pat. No. 4,063,956 discloses coating monoclinic bismuth vanadate with a first metal oxide hydrate layer (e.g., aluminum oxide hydroxide) and a second dense layer of amorphous silicon dioxide. In U.S. Pat. No. 4,115,141, bismuth vanadate is stabilized by coating with silicon dioxide or aluminum phosphate.
Combined oxide coatings are also described in U.S. Pat. No. 4,455,174, where bismuth vanadate pigments of the composition BiVO4.x Bi2MoO6.y Bi2WO6 (x=0.6-2.25, y=0-0.1) are coated first with zirconium dioxide and then with silicon dioxide. In U.S. Pat. No. 4,752,460, doped tetragonal bismuth vanadate pigments of the type (Bi,A) (V,D)O4 (A=Mg, Ca, Sr, Ba, Zn; D=Mo and/or W; molar ratio of A:Bi=0.1-0.4 and D:V=0-0.4) are coated first with silicon dioxide and then with aluminum oxide.
U.S. Pat. No. 5,123,965 describes coating doped tetragonal bismuth vanadate pigments with aluminum phosphate, calcium phosphate, titanium phosphate and mixtures of zinc phosphate and the phosphate of aluminum, of magnesium, of zirconium, of titanium or of calcium. Fluoride ions may be present during the coating with aluminum phosphate.
Fluoridic metal oxide coatings based on silicon dioxide, magnesium silicate and magnesium fluoride are finally known from EP-A-271 813, where bismuth vanadate pigments of the formula BiVO4.x Bi2MoO6 (x=0.2-0.25) are coated with this mixed layer and additionally with a wax layer.
However, the known coatings do not always lead to bismuth vanadate pigments having satisfactory properties.
It is an object of the present invention to provide bismuth vanadate pigments having good application properties, especially good stabilities, for example good weatherfastness.
We have found that this object is achieved by bismuth vanadate pigments comprising at least one coating containing calcium fluoride, bismuth oxyfluoride or a lanthanide fluoride or oxyfluoride or a mixture thereof.
This invention also provides for the use of these bismuth vanadate pigments for coloring paints, printing inks and plastics.
The bismuth vanadate pigments of the invention may be based on any known bismuth vanadate pigment, including those mentioned above. Further examples of suitable base pigments are the doped bismuth vanadate pigments described in EP-A-640 566 and DE-A-195 29 837.
The bismuth vanadate pigments of the invention are coated at least with a metal fluoride layer consisting essentially of calcium fluoride, bismuth oxyfluoride or a lanthanide fluoride or oxyfluoride, preferably lanthanum fluoride, lanthanum oxyfluoride, cerium fluoride, cerium oxyfluoride, yttrium fluoride or yttrium oxyfluoride. The fluorides (and/or oxyfluorides) mentioned may be present together in one and the same layer, but the separate application of layers each containing only one fluoride is preferred.
Particular preference is given to calcium fluoride layers and bismuth oxyfluoride layers, although calcium fluoride layers are preferably combined with further stabilizing coatings.
Thus, the metal fluoride coating of the invention may with advantage be combined with metal oxide coatings and/or metal phosphate coatings, in which case combinations of metal fluoride layers and metal oxide layers are preferred. In general, the layers are applied sequentially, but a certain degree of intermixing of the layers cannot be ruled out, especially in the case of the same layer type (fluoride, oxide or phosphate). Preferably, the metal fluoride coating(s) is or are applied to the bismuth vanadate pigment as the innermost layer in the case of multiple coating. However, the sequence of coats may also be changed.
Preferred materials for the metal oxide coatings are oxides and oxide hydrates of alkaline earth metals, especially magnesium, calcium, strontium and barium, of aluminum, silicon, tin, titanium, zirconium, hafnium, niobium, tantalum, zinc and of lanthanide metals, especially lanthanum, cerium and yttrium. Mixed oxides of these metals, especially the metal silicates, are particularly suitable. These compounds may likewise be present together in one and the same layer.
Examples of particularly preferred oxides are aluminum oxide, aluminum oxide hydrate, cerium dioxide and silicon dioxide and also the calcium silicates CaSiO3 and Ca2SiO5, of which CaSiO3 and silicon dioxide are most preferred. Where a metal oxide layer is present as outer layer, a silicon dioxide layer is particularly favorable.
Preferred materials for the metal phosphate coatings are the phosphates, especially the orthophosphates, of alkaline earth metals, especially magnesium and calcium, of zinc and of aluminum, which may also be present mixed in one and the same layer, this being preferred for the alkaline earth metals and zinc.
The bismuth vanadate pigments of the invention may have any number of coats. The number of coats is preferably within the range from one to four. Examples of particularly preferred coats are single coats of bismuth oxyfluoride and combination coats of calcium fluoride, calcium metasilicate and silicon dioxide or of bismuth oxyfluoride, calcium fluoride, calcium metasilicate and silicon dioxide. Further suitable combinations may be found in the Examples.
Depending on the particle size and the specific surface area of the bismuth vanadate used, the stabilized bismuth vanadate pigments of the invention generally contain from 2 to 40% by weight, preferably from 4 to 20% by weight of coating material, based on the weight of the coated pigment. The fluoride content is generally within the range from 0.05 to 10% by weight, preferably within the range from 1 to 5% by weight, based on the weight of the coated pigment.
The bismuth vanadate pigments of the invention are notable for their high stability, especially their very good weatherfastness and low photochromism. Photochromism is the reversible transformation of a compound into another of different color (absorption spectrum) due to visible or ultraviolet light. The measure of photochromism employed herein is the CIELAB xcex94E total color difference. The bismuth vanadate pigments of the invention also have excellent acid stability when coated with a bismuth oxyfluoride layer.
However, the bismuth vanadate pigments of the invention are not just convincing with regard to their stability, but surprisingly also have excellent color properties, especially high chroma and lightness.
To prepare the bismuth vanadate pigments of the invention, the coatings are advantageously precipitated wet-chemically onto the selected base pigment.
To deposit the metal fluoride layer, a suspension of the substrate (which can be an uncoated bismuth vanadate pigment or a bismuth vanadate pigment already coated with metal oxide or metal phosphate), a solution of a calcium, bismuth or lanthanide metal salt and a solution comprising fluoride ions are thoroughly mixed, preference being given to the use of aqueous solutions and suspensions.
Processwise it is possible to proceed in various ways: The substrate suspension can be introduced as initial charge and the calcium, bismuth or lanthanide metal salt solution and the fluoride ion solution added at the same time. However, it is also possible to introduce the fluoride ion solution as initial charge together with the substrate suspension and to add the calcium, bismuth or lanthanide metal salt solution, or vice versa to introduce the calcium, bismuth or lanthanide metal salt solution as initial charge together with the substrate suspension and to add the fluoride ion solution.
The pH of the mixture during the addition of the calcium, bismuth or lanthanide metal salt solution and/or the fluoride ion solution is advantageously maintained within the range from 2 to 11, preferably within the range from 5 to 9.
The temperature during the precipitation can be within the range from room temperature to the boiling point of the mixture (reflux temperature). A temperature within the range from 20 to 80xc2x0 C. is preferred.
When coating with metal oxide layers is desired, it is possible to proceed in a conventional manner by mixing the substrate suspension (uncoated or precoated bismuth vanadate pigment) with a preferably aqueous solution of a salt of the respective metal and precipitating the oxide or oxide hydrate onto the substrate while maintaining a pH which is customarily within the range from 3 to 10, preferably within the range from 5 to 9.
Metal phosphate layers can be similarly deposited in a likewise known manner by mixing the substrate suspension with the corresponding metal salt solution(s) and a phosphate ion solution while maintaining a pH within the range from 3 to 10 in general, but preferably within the range from 5 to 9.
To prepare the metal salt solutions required for the precipitation reactions, it is possible in principle to use any salt of the metals with inorganic or organic acids which is soluble in water (by addition of an acid, if necessary). Examples of preferred metal salts are calcium nitrate, calcium sulfate, calcium chloride, magnesium nitrate, magnesium sulfate, magnesium chloride, aluminum sulfate, aluminum nitrate, sodium aluminate, aluminum acetate, alkali metal silicates such as sodium silicate and potassium silicate, zinc nitrate, zinc sulfate, zinc chloride, bismuth nitrate, cerium nitrate, cerium ammonium nitrate, cerium sulfate, cerium chloride, lanthanum nitrate, lanthanum sulfate, lanthanum chloride, yttrium nitrate, yttrium sulfate and yttrium chloride.
The fluoride ion solutions are preferably prepared from alkali metal fluorides, ammonium fluorides or complex fluoride-containing salts. Examples of particularly suitable fluorides are: sodium fluoride, potassium fluoride, potassium hydrogendifluoride, ammonium fluoride, ammonium hydrogenfluoride, sodium tetrafluoroborate and ammonium tetrafluoroborate.
Examples of preferred phosphate ion solutions are solutions of alkali metal phosphates and hydrogenphosphates, especially sodium phosphate and potassium phosphate, and especially phosphoric acid.
After the last layer has been deposited, or on completion of the last addition, the suspension is generally stirred for from 1 to 5 h. The coated bismuth vanadate pigment may then, optionally after cooling to room temperature, be isolated in a conventional manner by filtration, washing and drying.
If desired, the coated bismuth vanadate pigment may be subjected to a grinding operation. Preference is given to a wet-grinding operation, which is preferably inserted after the washing of the pigment.
Tinctorially particularly useful bismuth vanadate pigments of high chroma and lightness are obtained on subjecting the pigment to a thermal treatment.
The point in time at which the thermal treatment is carried out is generally immaterial. It is possible to heat-treat either the uncoated base pigment in a conventional manner following its synthesis or the coated pigment after drying.
To heat-treat the dried coated pigment, it is generally heated to  greater than 300xc2x0 C., preferably to 350-700xc2x0 C., for 0.5-20 h.
The heat-treated coated pigment is advantageously then subjected to a wet-grinding operation. In this case, there is of course no need to grind the pigment after coating.
If desired, the bismuth vanadate pigments according to the invention may be additionally coated with organic additives in order, for example, that their dispersibility in paint systems may be improved.
Bismuth vanadate pigments of the invention are very useful for coloring paints, printing inks and plastics.